Method of heating steel ingots soaking pits and combustion system for performing said method

ABSTRACT

In a soaking furnace for heating metal parts, a burner works at full power during a heating-up period and at reduced power during a soaking period. Combustion gases are fed into the furnace with such velocity that the resulting turbulence causes uniform temperature distribution. When the burner is operated at reduced power, the total amount of fuel and air supplied is reduced but the speed of admission to the furnace is increased to maintain turbulence and uniform temperature distribution. The burner includes two concentric parts connected separately to common fuel air manifolds so that one part may be switched off thereby causing higher rate of flow to the other burner part.

United States Patent Pere et al.

[54] METHOD OF HEATING STEEL INGOTS SOAKING PITS AND COMBUSTION SYSTEMFOR PERFORMING SAID METHOD [72] Inventors: Carlo Pere, 2, Via Mura delIrato; Fulvlo Tornlch, 18, Via Bottini, both of Genoa, Italy [22] Filed;NOV. 10, I970 [21] Appl. No.: 88,281 v [30] Foreign Application PriorityData Nov. 15, 1969 Italy ..7448 A/69 [52] US. Cl. ..263/40 R, 263/52[51] Int.- Cl ..F27b 3/02 [58] Field of Search ..263/40, 43, 52

[56] References Cited UNITED STATES PATENTS Urquhart... ..26 3/52 [451 ISept. 5, 1972 2,776,827 I/ 1957 Graham ..263/52 Primary Examiner-John J.Camby Att0rneyBerman, Davidson 8:. Berman [57] ABSTRACT In a soakingfurnace for heating metal parts, a burner works at full power during'aheating-up period and at I reduced power during a soaking period.Combustion gases are fed into the furnace with such velocity that theresulting turbulence causes uniform temperature distribution. When theburner is operated at reduced,

power, the total amount of fuel and air supplied is reduced but thespeed of admission to the 'fumace is increased to maintain turbulenceand uniform temperature distribution. The burner includes two con;centric parts connected separately to commonfuel air manifolds so thatone part may be switched off thereby causing higher rate of flow to theother burner part p 10 wipf lfi PAIENTED ms? 5 m2 SHEET t [If 4 VENToes.

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METHOD OF HEATING STEEL INGOTS SOAKING PITS AND COMBUSTION SYSTEM FORPERFORMING SAID METHOD BACKGROUND OF THE INVENTION The invention relatesto a method of heating metal parts, particularly steel ingots or thelike in soaking furnaces, particularly pit type furnaces, in which thefurnace space is heated with the aid of a burner arrangement by thesupply of fuel and air for combustion in suitably regulated proportions.In such methods the most uniform possible temperature distribution inthe furnace space may be achieved through intensive turbulence of thecombustion gases, and the heating operation may consist of a heating-upperiod, in which the burner arrangement works at high heating rate andof a soaking period which follows and in which the burner arrangementworks at reduced heating rate throughthe reduction of the total amountof fuel and air'for combustion supplied, while retaining the same fuelto air ratio. The invention also relates to burner arrangements forcarrying out such methods.

The problem underlying the invention consists in developing a method ofthis kind which reduces the total heating time, permits economicaloperation of the soaking or pit type furnace, and in conjunction with ageneral increase in output of the furnace plant provides uniform soakingof the furnace charge to the desired final temperature, for examplerolling temperature, with the greatest possible accuracy.

SUMMARY OF THE INVENTION According to the invention this problem issolved by heating the furnace space walls during the heating-up periodto a temperature which is higher than the highest permissible surfacetemperature of the furnace charge, while at least in the startingportion of the following soaking period the speed of admission of thefuel and air for combustion into the furnace chamber is increased tosuch an extent that the turbulence and uniformity of distribution of thecombustion gases in the furnace space at least remain unchanged or areincreased.

In the method according to the invention the furnace walls are thusfirst, that is to say in the heating-up period with the burnerarrangement working at high or full power, heated to a temperature whichis higher than the maximum permissible surface temperature of thefurnace charge, that is to say than the temperature at which for examplethe surfaces of the steel ingots begin to melt. Heat at high temperatureis thus stored in the furnace space walls, while at the same time thestill relatively cold furnace charge is heated rapidly and effectivelyby the hot combustion gases. The burner arrangement is so constructedand designed, or is operated in such a manner, that in the entirefurnace space the most uniform possible temperature distribution isachieved, particularly through intensive turbulence of the combustiongases. The ratio of fuel to combustion air has a determined favorablevalue adapted to requirements in each particular case. In the followingsoaking period the power of the burner arrangement is reduced bythrottling the total amount of fuel and air for combustion supplied,while retaining the same ratio of fuel to air for combustion. Thus onthe one hand the heat supplied by the burner arrangement and on theother hand the volume of combustion gases produced are correspondinglyreduced. Consequently the furnace space walls, which were superheatedduring the heating-up period to a higher temperature than the maximumpermissible surface temperature of the furnace charge, now give up theirstored heat to the furnace charge and thereby provide the heat stillrequired for the soaking of the furnace charge even though the power ofthe burner arrangement is now throttled. At the same time the speed ofadmission of the fuel and air for combustion entering the furnace spaceis increased to such an extent that the turbulence of the hot combustiongases is retained practically unchanged as in the heating-up period, orpossibly even increased, even though the volume of combustion gases hasnow been reduced. Despite the reduced power of the burner arrangement,the same most uniform possible temperature distribution is thus on theone hand retained in the entire furnace space, while on the other handthe transmission to the furnace charge of the heat stored in the furnacespace walls is assisted, accelerated, and made uniform, since thetransfer of heat is also, or mainly, effected by convection and notsolely by pure radiation. As a final result, very rapid and uniformheating of the furnace charge, in conjunction with particularlyeconomical operation of the burner arrangement or of the furnace plant,is achieved.

Any desired gaseous fuel, for example generator gas, blast furnace gas,coke oven gas, and the like, or any desired liquid or solid fuel, whichhowever must be capable of being brought into a gaseous or gas-likecondition, for example an atomized or gasified liquid fuel or a mixtureof pulverized coal and air, or the like, may be used as fuel forcarrying out the method according to the invention.

The speed of admission of the fuel and air for combustion into thefurnace space may have any desired absolute value both during theheating-up period and in the soaking period. The only important thing isthat this speed of admission, in conjunction with the construction ordesign of the burner arrangement, should ensure the greatest possibleturbulence and the most uniform distribution of the combustion gases inthe furnace space during the heating-up period with the burnerarrangement operating at full power, and that in the following soakingperiod, with the burner operation operating at reduced power, this speedshould be increased to such an extent that in consequence of thecorrespondingly higher turbulence of the combustion gases, at least thesame uniform distribution of the combustion gases is maintained.

For the purpose of carrying out the method, the invention provides aburner arrangement which is characterized by at least two burners, eachwith a fuel supply pipe and an air supply pipe, and also by controlmeans, controlled automatically in dependence on the temperature in thefurnace space, for throttling the fuel and air supply pipes of oneburner and simultaneously correspondingly increasing the pressure in thefuel and air supply pipes of the other burner, at least in the initialportion of the soaking period, while the ratio of the fuel to air forcombustion remains constant for both burners. In the heating-up periodboth burners may work practically with full power, the total amount offuel and air for combustion supplied to the two burners corresponding toheat requirements for heating the furnace space walls to a predeterminedmaximum temperature which is higher than the maximum permissible surfacetemperature of the furnace charge. The pressure in the fuel and airsupply pipes for both burners, and consequently also the speed ofadmission of the fuel and air for combustion into the furnace space,which is dependent thereon, are so high that the turbulence of thecombustion gases in the furnace space ensures the desired uniformtemperature distribution. After the maximum temperature of the furnacespace walls has been reached, the fuel and air supply pipes of oneburner are automatically throttled progressively by the control meansresponding to the furnace space temperature, until the burner inquestion is completely disconnected or stopped. The entire amount offuel and air for combustion introduced into the furnace space, andconsequently both the heat supplied and the volume of the combustiongases produced, are thereby reduced. At the same time however thepressure in the fuel and air supply pipes of the other burner arelikewise automatically increased to such an extent that the resultinghigher speed of admission of the fuel and air for combustion bringsabout the same or greater turbulence of the combustion gases in thefurnace space and maintains the same or greater uniformity ofdistribution of these combustion gases and consequently also of thetemperature.

According to another advantageous feature of the invention the twoburners are combined in a common burner head and disposed coaxiallyrelative to each other, with concentric annular outlet nozzles for fueland air. Through this arrangement, in conjunction with a simple, compactconstruction, the same spatial or geometrical conditions are retainedfor the formation of turbulence of the combustion gases in the furnacespace both in the heating-up period when both burners are operatingsimultaneously and in the following soaking period when a single burneris operating.

According to another advantageous feature of the invention, the increasein pressure in the fuel and air supply pipes of one burner in dependenceon the throttling of the fuel and supply pipes of the other burner canbe achieved in a particularly simple manner by connecting on the onehand the fuel supply pipes and on the other hand the air supply pipes ofboth burners through respective fuel and air manifolds to common fueland air blowers, while the fuel and air supply pipes of one burner areprovided with simultaneously operable throttle elements. In thisconstruction, at the end of the heating-up period only the throttleelements in the fuel and air supply pipes of one burner need be closedsimultaneously, whereby the pressure in the fuel and air supply pipes ofthe other burner is automatically increased while the fuel and airblowers continue to run unchanged.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a burner arrangement forcarrying out the method according to the invention, illustrateddiagrammatically,

FIG. 2 is a longitudinal section of the burner head with two coaxialburners,

FIG. 3 is an elevation of the burner head, viewed from the right-handside in FIG. 2, and

FIG. 4 shows some characteristic curves of the method according to theinvention during the heatingup and soaking periods.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustratesdiagrammatically a pit type furnace 1 with a cover plate 101 and wastegas offtake 3 leading into a chimney 2. The pit type furnace l is heatedwith the aid of a burner arrangement which is illustrateddiagrammatically in FIG. 1 and of which a preferred form of constructionis shown in FIGS. 2 and 3. This burner arrangement consists of at leastone burner head 4 provided in the side wall of the pit type furnace land associated with two burners disposed coaxially one around the otherand operated with a mixture of gaseous fuel, for example, coke oven gasand air for combustion.

In the example of embodiment illustrated the two burners consist of anair chamber 5 and three tubes 6, 7, and 8 disposed in said air chambercoaxially around one another and spaced radially apart. The air chamber5 is connected on the one hand to an air supply pipe 9 and on the otherhand to a ring of air outlet nozzles 10 disposed in the burner head. Theair outlet nozzles 10 are inclined convergingly in relation to thelongitudinal axis of the burner (as shown in FIG. 2) and at the sametime are also directed obliquely to the respective radial planes, sothat the axes of the nozzles do not pass through the axis of the burneras can be seen in particular from FIG. 3. The outer tube 6 is joined toa fuel supply pipe 11 and leads into a central outlet aperture 12 in theburner head 4. The other two coaxial pipes 7 and 8 likewise lead intothe central outlet aperture 12 of the burner head 4, the intermediatetube 7 being connected by an elbow 13 to an air supply pipe 14 and theinnermost tube 8 to a fuel supply pipe 15. The air chamber 5, with theair outlet nozzles 10, and the outer fuel tube form a first, outerburner associated with the air and fuel supply pipes 9 and 1 1. Theintermediate air tube 7 and the innermost fuel tube 8 form a second,inner burner with the associated air and fuel supply pipes 14 and 15.

The air supply pipes 9, 14 of both the outer burners 5, 6 and the innerburner 7, 8 are connected through an air manifold 16 to a common airblower 17, as illustrated particularly in FIG. 1. The fuel supply pipes11 and 15 of the two burners 5, 6 and 7, 8 are likewise connected by afuel manifold 18 to a common fuel blower l9. Throttle elements 20 and 21respectively are provided in the air supply pipe 9 and in the fuelsupply pipe 1 1 of the outer burner 5, 6. The air and fuel manifolds 16and 18 respectively are likewise each provided with a throttle element22 and 23 respectively. The throttle elements 20, 21, 22, 23 areoperated by electrical adjusting motors 120, 121, 122 and 123respectively. The adjusting motors and 122 of the throttle elements 20and 22 in the air supply pipe 9 of the outer burner 5, 6 and in the airmanifold 16 associated with the two burners 5, 6 and 7, 8 are controlledby a temperature sensor 25 responding to the temperature of the innerwall of the pit furnace 1, with the aid of a thermoelectric transducer25 and a control device 26, which is known in itself.

The suction pipes 117 and 119 of the air and fuel blowers 17 and 19respectively each contain a flow measuring device 27 and 29respectively, these devices being known in themselves. These measuringdevices 27, 29 are each connected by a respective transducer 127 and 129to a controller 28, which is likewise known in itself and which controlsthe adjusting motor 121 of the throttle element 21 in the fuel supplypipe 11 of the outer burner 5, 6 and the adjusting motor 123 of thethrottle element 23 in the fuel manifold 18 associated with the twoburners 5, 6 and 7, 8.

The controller 28 is constructed in a manner known in itself so that inthe event of fluctuations of the flows in the suction pipes 1 17, l 19of the air and fuel blowers l7, l9, and particularly in theevent ofvariations of the air flow in the suction pipe 117 of the air blower 17,it operates the throttle elements 21, 23 so as to maintain apredetermined, adjustable and readjustable fuel/air for combustion ratioin both the coaxial burners.

The method of heating metal parts, particularly steel ingots, in the pittype furnace l, which can be carried out with the burner arrangementaccording to FIGS. 1 to 3,will now be described with the aid of FIG. 4.FIG. 4 shows the following characteristic curves plotted against time T:

Curve I-I: Pressure of fuel and air for combustion in the outer burner5, 6, in millimeters of water column.

Curve K: Pressure of fuel and air for combustion in the inner burner 7,8, in millimeters of water column.

Curve L: Total inflow of fuel and air for combustion in both burners 5,6 and 7, 8 as a percentage of the total fuel and air for combustioninflow with both burners at full power.

Curve M: Temperature of combustion gases in the pit furnace space, in C.

Curve N: Temperature of inner wall of pit furnace space, in C. Theheating-up period A extends to the point of time T1 and then the soakingperiod D commences. S indicates the maximum permissible surfacetemperature of the steel ingots, that is to say the temperature at whichthe surfaces of the ingots begin to melt.

At the beginning of the heating-up period A the throttle elements 20, 21in the air and fuel supply pipes 9, ll of the outer burner 5, 6 areopen. Both burners 5, 6 and 7, 8 work at full power. The throttleelements 22 and 23 in the air and fuel manifolds 16, 18 are opened tosuch an extent that on the one hand the adjusted ratio of fuel to airfor combustion and on the other hand the total inflow of fuel and airfor combustion required for maximum power operation of both burners 5, 6and 7, 8 are achieved. The pressure in the air and fuel manifolds 16, 18and in the air and fuel supply pipes 9, 11 and 14, branching offtherefrom and feeding the two burners 5, 6 and 7, 8 respectively, is sohigh that the admission speed of the fuel and air for combustionentering the pit type furnace 1, which is pressure dependent, iscorrespondingly high and gives rise to great turbulence of thecombustion gases and consequently uniform distribution of the combustiongases and of the temperature in the furnace space.

In the heating-up period A the still relatively cold steel ingots absorbheat from the hot combustion gases, while at the same time the walls ofthe pit type furnace l are heated. The heating-up period A lasts untilthe walls of the pit type furnace reach a temperature N] which is higherthan the maximum permissible surface temperature S of the steel ingots.The temperature sensor 24 responds to this maximum temperature N1 of thefurnace space walls and by means of the control device 26 and theadjusting motor 120 effects the gradual closing of the throttle element20 in the air supply pipe 16 of the outer burner 5, 6. The measuringinstrument27 disposed in the suction pipe 117 of the air blower 17responds to the reduction of air flow produced by the closing of thethrottle element 20. The controller 28 consequently closes the throttleelement 21, by means of the adjusting motor 121. It does thissimultaneously with closure of the air throttle element 20 and maintainsthe required adjusted fuel/air for combustion ratio. The closing of thethrottle elements 20 and 21 in the air and fuel supply pipes 9 and 11respectively of the outer burner 5, 6 starts at the moment T1 at the endof the heating-up period A and is terminated at the moment T2 in thefollowing soaking period D. The pressure of the air for combustion andfuel in the outer burner 5,6 consequently gradually drops to zero in theperiod Tl-T2, as the portion H1I-l2 of curve H shows. Consequently,withinkthis period Tl-T2 the total inflow of fuel and air for combustionalso drops progressively from the percent value L1 corresponding to fullpower of both burners 5, 6 and 7, 8 to a value L2 corresponding only tofull power of the inner burner 7, 8 which continues to operate. At thesame time the pressure of the fuel and of the air for combustion in theinner burner 7, 8 gradually rises, as the portion Kl-KZ of the curve Kshows, since with increasing throttling of the air and fuel supply pipes9, 11 of the outer burner 5, 6 the corresponding pressure losses falloff and the inner burner 7, 8 now receives the entire delivery pressureof the air and fuel blowers 17, 19. The pressure rise in the innerburner 7, 8 corresponds to an increase of the speed of admission of theair for combustion and fuel into the pit type furnace 1. This increasedspeed of the air for combustion and fuel feeding the inner burner 7, 8is so great that even though the total volume of combustion gasesproduced in now smaller it produces approximately the same intensiveturbulence of the combustion gases in the furnace l as in the heating-upperiod A, or even produces still greater turbulence and consequently,despite the reduced power of the burners, ensures uniform distributionof the combustion gases and of the temperature in the furnace space.

At the moment Tl, that is to say at the end of the heating-up period,the temperature N1 of the furnace space walls is higher and thetemperature M1 of the combustion gases is lower than the maximumpermissible surface temperature S of the steel ingots. In consequence ofthe throttling of the outer burner 5, 6 and of the correspondingreduction of the supply of heat, the superheated furnace space walls nowgive their stored heat to the steel ingots. The maintenance of theintensive turbulence of the combustion gases in the furnace spacesubstantially assists the transfer of heat from the furnace space wellsto the steel ingots, since it causes transfer of heat through convectionand not through pure radiation alone. The temperature of the combustiongases consequently rises slightly further and is stabilized at thetemperature value M2, which is still lower than the maximum permissiblesurface temperature S of the steel ingots. The temperature of thefurnace space walls on the other hand retains its maximum value N1 for arelatively short time and then gradually declines.

In'the soaking period D so much heat is supplied to the steel ingots,partly from the superheated furnaced space walls and partly from theinner burner 7, 8 which continues to operate, that all of them attainthe desired uniform final temperature, for example the prescribedrolling temperature, and at the same time are uniformly soaked. When thetemperature of the steel ingots approaches the desired finaltemperature, that is to say approximately at the moment T2, the throttleelement 22 in the air manifold 16 is gradually closed with the aid ofthe control device 26 and the adjusting motor 122. This is brought aboutby a control pulse of a temperature sensor, for example through thetemperature sensor 24 responding to the temperature of the furnace spacewalls. The controller 28, which maintains unchanged the adjusted ratioof air for combustion to fuel, simultaneously closes by means of theadjusting motor 123 the throttle element 23 in the fuel manifold 18. Theinflow of fuel and air for combustion drops further from the value L2corresponding to the full power of the inner burner 7, 8, while at thesame time the pressure of the fuel and of the air for combustion in theinner burner 7, 8 drop from the maximum value L2 which it had attained.

The main advantage of the method according to the invention consists inthat the furnace charge, for example the steel ingots disposed in thepit type furnace l, is heated uniformly throughout to the desiredtemperature in a substantially shorter time than by known methods, whilethe burner arrangement proposed according to the invention as beingpreferable for carrying out this method has a particularly simple,compact, operationally reliable, and economical construction.

The invention is naturally not restricted to the example of embodimentillustrated and described, but within the framework of the generalprinciple of the invention various modifications are possible. Inparticular, other burner arrangements may be used for carrying out themethod according to the invention. The burner arrangement proposed inaccordance with the invention may also be modified in respect of itsconstruction or operation. Thus, for example, at the end of theheatingup period A the inner burner 7, 8 may be :throttled or put out ofaction and the outer burner 5, 6 kept in operation. For this purpose itis merely necessary for the throttle elements 20, 21 to be disposed inthe air and fuel supply pipes 14 and respectively of the inner burner 7,8 instead of in the supply pipes 9, 11 of the outer burner.

We claim:

1. A soaking furnace including furnace walls enclosing a furnace spaceto contain an ingot to be treated and a burner arrangement connectedthereto, comprising at least two burners, each of which is provided witha fuel supply pipe and an air supply pipe, and control means controlledautomatically in dependence on the temperature in the furnace space forthrottling the air and fuel supply pipes of one burner and causingsimultaneous corresponding increaseof pressure in the air and fuelsupply pipes of the other burner at least in the initial portion of thesoaking period, the ratio of fuel to air for combustion remainingconstant for both burners.

2. A burner arrangement according to claim 1, in which the two burnersarecombined in a common burner head and are disposed coaxially aroundone another with concentric annular outlet nozzles for fuel and air.

3. A burner arrangement according to claim 2, in which on the one handthe air supply pipes and on the other hand the fuel supply pipes of thetwo burners are each connected by an air manifoldand a fuel manifold tocommon air and fuel blowers, and that the air and fuel supply pipes ofone burner are provided with simultaneously operable throttle elements.

4. A burner arrangement according to claim 3, in which the throttleelement in the air supply pipe of one burner is controlled by a controldevice responding to the temperature in the furnace space and thethrottle element in the fuel supply pipe of the same burner iscontrolled by a controller maintaining an adjusted ratio of fuel to air.

5. A burner arrangement according to claim 4, in which the air and fuelmanifolds common to the two burners are each provided with a throttleelement downstream of the respective air and fuel blowers, and thesethrottle elements can be operated simultaneously while retaining thesame ratio of fuel to air.

6. A burner arrangement according to claim 5, in which the throttleelement in the air manifold is controlled by a control device respondingto the temperature in the furnace space or to the temperature of thefurnace charge, and the throttle element in the fuel manifold iscontrolled by a controller maintaining an adjusted ratio of fuel to air.

7. A burnerarrangement according to claim 6, in which the said controldevice has a temperature sensor responding to the temperature of thefurnace space walls.

8. A burner arrangement according to claim 7, in which the saidcontroller for maintaining the adjusted ratio of fuel to air forcombustion is provided with two measuring instruments responding to theflow and disposed in the suction pipes of the air and fuel blowersrespectively.

9. A method of heating metal ingots in a soaking furnace of the typeincluding a pair of concentric burners having separate individual fueland air supply systems, the steps of heating the soaking furnace wallsto a temperature of the maximum permissible surface temperature of themetal ingot by initially operating both burners at their maximumcapacity producing an intense turbulence of the combustion gases in thefurnace space, reducing the total fuel and air supplied to the burnersduring a soaking period while simultaneously increasing the speed ofadmission of the fuel and air for combustion into the furnace space toan extent such that the turbulence of the combustion gases in thefurnace space does not initially decrease.

10. The method of claim 9 wherein the steps of reducing the total fueland air supplied to the burners while simultaneously increasing thespeed of admission of the fuel and air for combustion into the furnacespace includes the steps of reducing the fuel and air supplied throughone of said burners while simultaneously increasing the speed ofadmission of the fuel and air through the other of said burners.

1. A soaking furnace including furnace walls enclosing a furnace spaceto contain an ingot to be treated and a burner arrangement connectedthereto, comprising at least two burners, each of which is provided witha fuel supply pipe and an air supply pipe, and control means controlledautomatically in dependence on the temperature in the furnace space forthrottling the air and fuel supply pipes of one burner and causingsimultaneous corresponding increase of pressure in the air and fuelsupply pipes of the other burner at least in the initial portion of thesoaking period, the ratio of fuel to air for combustion remainingconstant for both burners.
 2. A burner arrangement according to claim 1,in which the two burners are combined in a common burner head and aredisposed coaxially around one another with concentric annular outletnozzles for fuel and air.
 3. A burner arrangement according to claim 2,in which on the one hand the air supply pipes and on the other hand thefuel supply pipes of the two burners are each connected by an airmanifold and a fuel manifold to common air and fuel blowers, and thatthe air and fuel supply pipes of one burner are provided withsimultaneously operable throttle elements.
 4. A burner arrangementaccording to claim 3, in which the throttle element in the air supplypipe of one burner is controlled by a control device responding to thetemperature in the furnace space and the throttle element in the fuelsupply pipe of the same burner is controlled by a controller maintainingan adjusted ratio of fuel to air.
 5. A burner arrangement according toclaim 4, in which the air and fuel manifolds common to the two burnersare each provided with a throttle element downstream of the respectiveair and fuel blowers, and these throttle elements can be operatedsimultaneously while retaining the same ratio of fuel to air.
 6. Aburner arrangement according to claim 5, in which the throttle elementin the air manifold is controlled by a control device responding to thetemperature in the furnace space or to the temperature of the furnacecharge, and the throttle element in the fuel manifold is controlled by acontroller maintaining an adjusted ratio of fuel to air.
 7. A burnerarrangement according to claim 6, in which the said control device has atemperature sensor responding to the temperature of the furnace spacewalls.
 8. A burner arrangement according to claim 7, in which the saidcontroller for maintaining the adjusted ratio of fuel to air forcombustion is provided with two measuring instruments responding to theflow and disposed in the suction pipes of the air and fuel blowersrespectively.
 9. A method of heating metal ingots in a soaking furnaceof the type including a pair of concentric burners having separateindividual fuel and air supply systems, the steps of heating the soakingfurnace walls to a temperature of the maximum permissible surfacetemperature of the metal ingot by initially operating both burners attheir maximum capacity producing an intense turbulence of the combustiongases in the furnace space, reducing the total fuel and air supplied tothe burners during a soaking period while simultaneously increasing thespeed of admission of the fuel and air for combustion into the furnacespace to an extent such that the turbulence of the combustion gases inthe furnace space does not initially decrease.
 10. The method of claim 9wherein the steps of reducing the total fuel and air supplied to theburners while simultaneously increasing the speed of admission of thefuel and air for combustion into the furnace space includes the steps ofreducing the fuel and air supplied through one of said burners whilesimultaneously increasing the speed of admission of the fuel and airthrough the other of said burners.